If an attached encrypted volume is failed to detach from the instance
when deleting that instance, user can not delete that instance at all.
This patch adds 4 in check_exit_code when executing `cryptsetup remove`
command to eat that exception.
PS: exit_code = 4 indicate ENODEV error which means no device(also includes
the crypt device inactive).
Closes-Bug: #1482066
Change-Id: I12e2a52068850528a4bd68486344b74eb9b82c88
bafc0f8d9c